Vapour compression refrigerating systems can be arranged so that the condensed liquid refrigerant coming from the condenser at high pressure is sub-cooled to an intermediate temperature before being fed to an expansion device. Sub-cooling has the benefit of increasing the refrigerating effect per unit mass of the circulating refrigerant. This will improve the efficiency of the system provided the additional capacity produced is greater than the power increase required to produce it.
Systems which use this effect include two-stage systems with intermediate cooling and liquid pre-cooling, two-stage systems without intercooling but with liquid pre-cooling (such systems are generally known as “economised” systems) and single-stage screw compressor systems which draw a proportion of the refrigerant flow into an “economiser” port as vapour so that the remainder of the refrigerant flow is sub-cooled to economiser pressure